1. Field of the Invention
The present invention relates to a lithographic printing plate precursor and a lithographic printing method thereof.
2. Description of the Related Art
A lithographic printing plate comprises an oleophilic image area which receives an oil-based ink at the printing step and a hydrophilic non-image area (non-ink-receptive area) which receives a fountain solution at the printing step. Lithographic printing is a printing method utilizing the repulsion between water and oil-based ink. The surface of the lithographic printing plate is made different in ink affinity from area to area. An ink is then attached to the image area alone. The ink is then transferred to printing material (e.g., paper) to effect printing.
In order to prepare lithographic printing plates, a lithographic printing plate precursor (PS plate) comprising an oleophilic photo sensitive resin layer (image-forming layer) provided on a hydrophilic support has heretofore been widely used. The lithographic printing plate precursor is exposed to light through an original (e.g., lithographic film). The image-forming layer on the non-image area is then dissolved away with an alkaline developer or organic solvent with the image-forming layer on the image area left undissolved. In this manner, the surface of the hydrophilic support is exposed to obtain a lithographic printing plate.
The related art process for the production of a lithographic printing plate precursor requires a step of dissolving the non-image area away with a developer after exposure. The recent technical assignment in the art is to eliminate or simplify a wet process which is additionally conducted as in the development step. In recent years, the disposal of waste liquid discharged from the wet process has become a great concern to the entire industry taking into account the global environment. With such an environmental problem, the demand for elimination of wet process has been growing more and more.
As a simple plate-making method there has been proposed a method called on-the-machine development, method which comprises removing the non-image area from the exposed lithographic printing plate precursor on the printing machine to obtain a lithographic printing plate. This method involves the use of an image-forming layer that allows the removal of the non-image area from the lithographic printing plate precursor at an ordinary printing step.
Specific examples of the on-the-machine development method include a method involving the use of a lithographic printing plate precursor comprising an image-forming layer capable of being dissolved or dispersed in a fountain solution, ink solvent or an emulsion of fountain solution and ink, a method involving the dynamic removal of an image-forming layer by contact with the rollers or blanket cylinder of the printing machine, and a method involving the dynamic removal of an image-forming layer by contact with the rollers or blanket cylinder of the printing machine after the reduction of the cohesive force of the image-forming layer or the adhesion between the image-forming layer and the support by the penetration of fountain solution, ink solvent, etc.
The term “development step” as used herein is meant to indicate a step of allowing the lithographic printing plate precursor to come in contact with a liquid (normally an alkaline developer) in an apparatus other than printing machine (normally an automatic developing machine) to remove the area unexposed to infrared laser beam from the lithographic printing plate precursor so that the surface of the hydrophilic support is exposed. The term “on-the-machine development” as used herein is meant to indicate a method and step of allowing the lithographic printing plate precursor to come in contact with a liquid (normally a printing ink and/or fountain solution) in a printing machine to remove the area unexposed to infrared laser beam from the lithographic printing plate precursor so that the surface of the hydrophilic support is exposed.
In recent years, on the other hand, a digitization technique involving electronic processing, storage and output of image data using computer has been widely spread. Various new image output methods that cope with this digitization technique have been put to practical use. With this technical trend, a computer-to-plate technique has been noted which comprises tracing a lithographic printing plate precursor with a highly convergent radiation such as laser beam having digitized image data carried thereon to produce a lithographic printing plate directly without lithographic film. Accordingly, one of important technical assignments is to obtain a lithographic printing plate precursor adapted for such a technique.
As a lithographic printing plate precursor capable of being exposed by tracing, one comprising a hydrophilic support and an oleophilic photosensitive resin layer which is provided on the hydrophilic support and containing a photosensitive compound capable of generating an active species such as a radical or a Brensted acid when exposed to a laser light is proposed and already introduced in the market. A nega-type lithographic printing plate is obtained by tracing such a lithographic printing plate precursor with laser using digital information to generate an active species, by the action of which a physical or chemical change is triggered in the photosensitive layer to cause insolubilization of the layer, and successively subjecting the exposed predursor to development processing. In particular, a lithographic printing plate precursor comprising a hydrophilic support, a photopolymerizable photosensitive layer provided thereon and containing a photopolymerization initiator excellent in photosensitive speed, an addition-polymerizable ethylenically unsaturated compound and an alkaline developer-soluble binder polymer, and, if necessary, an oxygen-blocking protective layer exhibits desirable printing performance since it has many advantageous features such as excellent productivity, simple development process, and preferable resolution as well as ink adhesion.
As an on-the-machine developable lithographic printing plate, Japanese Patent 2,938,397 describes a lithographic printing plate precursor comprising a hydrophilic support and an image-forming layer provided thereon and containing hydrophobic thermoplastic polymer particles dispersed in a hydrophilic binder. This Japanese Patent 2,938,397states that the above-described lithographic printing plate precursor can be on-press developed with a printing ink and/or a fountain solution after it is exposed to an infrared laser whereby an image is formed due to the thermal fusion of the hydrophobic thermoplastic polymer particles, and then mounted on a cylinder of a press machine.
The method of forming an image only by simple thermal fusion of fine particles as described above has disadvantage of insufficient printing durability due to an extremely low image strength (poor adhesion to the support) in spite of good on-press developability.
Further, a lithographic printing plate precursor comprising microcapsules having a polymerizable compound encapsulated therein incorporated in a hydrophilic support has been proposed (see, e.g., JP-A-2001-277740 and JP-A-2001-277742).
Moreover, a lithographic printing plate precursor comprising a photosensitive layer containing an infrared absorbent, a radical polymerization initiator and a polymerizable compound provided on a support has been proposed (see, e.g., JP-A-2002-287334).
Thus, the method involving the use of polymerization reaction provides an image area having a higher chemical bond density and hence a relatively higher image strength than that formed by heat fusion of polymer particles but leaves something to be desired all in on-the-machine developability, press and polymerization efficiency (sensitivity) from the practical standpoint of view and thus has not yet been practically used.